To increase the areal storage density of a magnetic recording device, the recording layer thereof may be provided with smaller and smaller individual magnetic grains. This reduction in grain size soon reaches a “superparamagnetic limit,” at which point the magnetic grains become thermally unstable and incapable of maintaining their magnetization. The thermal stability of the magnetic grains can be increased by increasing the magnetic anisotropy thereof (e.g., by utilizing materials with higher anisotropic constants). Increasing the magnetic anisotropy of the magnetic grains, however, increases their coercivity and therefore requires a stronger magnetic field to change the magnetic orientation of the grains (e.g., in a write operation).
Energy-assisted magnetic recording (EAMR) is used to address this challenge. In an EAMR system, a small spot where data is to be written is locally heated to reduce the coercivity of the magnetic grains therein for the duration of the write operation, thereby allowing materials with increased magnetic anisotropy to be used, and greater areal storage density to be exploited. In EAMR approach, a semiconductor laser diode is normally used as a light source and coupled to a planar waveguide which serves as light delivery path. A grating structure may be used to couple the laser light into the waveguide. Design challenges for these grating structures include improving their coupling efficiency and the difficulty in aligning a light source for high volume manufacturing processes. The coupled light is then routed to a near field transducer by which the optical energy is provided to a small spot on the recording media a few tens of nanometers (nm) in size.
The overall light coupling efficiency is important in this approach for a number of reasons. First, it ensures that sufficient energy is delivered to the media so that a sufficient thermal change is achieved for the recording operation. Second, it allows for a lower power (i.e., less expensive) light source to be used. Finally, with improved efficiency, the total power consumption of the EAMR can be reduced, reducing reliability issues associated with high operating temperatures. Unfortunately, many EAMR heads have such poor coupling efficiency that they require costly high-power light sources and suffer from many heat-related reliability issues.